1. Field of the Invention
The present invention related to a roll control device of a vehicle such as an automobile, and more particularly, to a device which controls a rolling of a vehicle body based upon a selective braking of a wheel or wheels.
2. Description of the Prior Art
It is a still standing desire that a vehicle such as an automobile is protected by the art of vehicle stability control from the rolling down due to a too sharp steering, particularly with those vehicles called recreation vehicles which are often driven like a slalom in the off-road open places.
As a matter of physical principle, the rolling of a vehicle during a turn running can be suppressed by decelerating the vehicle or increasing the radius of the turning curve, i.e. drifting out the vehicle, or doing both of these, so that the lateral centrifugal force proportional to the ratio of the square of the running speed to the radius of the running curve is decreased. In the four-wheeled vehicles handled as an object of the art of vehicle stability control, equipped with a brake system applicable a controlled braking to each of the four wheels independently of the other, it is known that the driftout of the vehicle is available by applying a braking to the front wheel serving at the outside of the turn, based upon the principle that the tire grip force available in the lateral direction is decreased in a reversal complement to the increase of the longitudinal tire grip force effected by the braking, since the vector addition of the longitudinal and lateral tire grip forces is restricted within the so-called friction circle.
Further, in the four-wheeled vehicle which has a vehicle body supported on pairs of front and rear wheels via an elastic suspension system, the rolling of the vehicle has a dynamic aspect that it is a rolling movement around a longitudinal roll axis as well as a static aspect that the lateral centrifugal force acting to the vehicle body at its center of gravity is statically balanced by the elastic force applied to the vehicle body from the elastic suspension proportional to the product of the rolling angle and the modulus of elasticity of the suspension system.
In Japanese Patent Laid-open Publication 63-116918, there is described a roll control device for a vehicle, wherein a parameter called "vehicle condition" is calculated based upon a signal received from a roll estimation sensor, and the vehicle speed is controlled to be decreased before the vehicle condition reaches a limit value.
In Japanese Patent Laid-open Publication 6-297985, there is described a roll control device for a vehicle, wherein a parameter called "roll down reference value" is calculated based upon the height of the center of gravity of the vehicle and a roll angle of vehicle body, and the vehicle is decelerated when the roll down reference value exceeds a limit value.
In Japanese Patent Laid-open Publication 10-81215 and German Patent Laid-open Publication DE 19632943 A1 corresponding thereto, there is described a method of driving a vehicle, wherein when the value of at least one rolling tendency indication parameter such as the lateral acceleration, a time derivative of the lateral acceleration, etc. exceeds a threshold value, the wheels serving at the outside of the turn are braked, so as to let the wheels slide outside of the turn according to the principle of reversal complement of the longitudinal tire grip force and lateral tire grip. (According to the German version distinctive of singularity and plurality, the braking is applied to a plurality of wheels serving at the outside of the turn.)
It is noted in all of the above three publications that, although the roll control is described to trigger the braking by each watching parameter exceeding its threshold value, it is not described how the braking is progressed. However, based upon the conventional standard concept of such a parameter-based automatic control, it is guessed that the parameter selected for watching to start the roll suppress control is watched in continuity to control the progress of the braking. Since the roll suppress controls of the above-mentioned publications do not appear to stop the vehicle when the watched parameter has once exceeded the threshold value, the braking applied independently of the driver's intention will have to be released as soon as possible according to the subsidence of the rolling. Therefore, the braking will be applied under the control of the parameter which triggered the braking.
However, as described above, in the rolling of a four-wheeled vehicle, there are both dynamic and static aspects. In more detail, when the rolling state is judged, for example, by the lateral acceleration of the vehicle body, the urgency for a roll suppress control is different for two such turning states showing a common lateral acceleration that in one the vehicle body is under a static balance between the lateral centrifugal force and the elastic suspension force and in the other the vehicle body is still rolling toward outside of the turn. Therefore, in order to increase the theoretical precision of the roll control, the dynamic aspect of the rolling will have to be weighted. However, as a matter of actual practice, the precision of detection of dynamic parameters is generally inferior to that of static parameters, particularly when the detection incorporates the detection of the parameter trespassing a threshold value. Therefore, although the dynamic aspect is interested in the roll suppress control, it is hesitated to employ a dynamic parameter such as a time derivative of the roll angle as the parameter to trigger and control the roll suppress braking.